1. Field of the Invention
The present invention relates to a diffraction optical element, and in particular, to a diffraction optical element with a stacked structure having a grading structure, where luminous flux in an operating wave-length range is concentrated in a specific degree, and its production method.
2. Related Background Art
Conventionally, a chromatic aberration of an optical system is corrected by combining optical elements consisting of transparent materials whose dispersions are different from each other. Moreover, how to correct the chromatic aberration by using a diffraction optical system is disclosed in SPIE Vol. 1354, 24-37.
By the way, at the time of adding a diffraction surface that has a diffraction efficiency to the optical system that has a spectral characteristic, it is important to keep the diffraction efficiency high in an operating wavelength range. However, in a diffraction type optical system, light with a degree other than a design degree has a larger angle of diffraction as the degree is apart from the design degree, and the difference of focal distances becomes larger. Hence, this appears as defocus, and when light source with particularly high brightness exists, a side lobe may arise.
On the other hand, by constituting a diffraction optical element with a stacked structure that has multiple layers composed of two or more layers, remarkable enhancement can be expected in quality of the image and information, and it becomes possible to enhance optical performance. Such a diffraction optical element with a stacked structure that has two or more of layers is formed by staking and bonding a plurality of optical members that have each diffraction grating. Moreover, such an optical member is made by separating a compound from a molding die after dropping a compound on a molding die where a pattern corresponding to a diffraction grating is formed and transcripting the pattern to the compound.
A well-known method of forming a pattern on the surface of a molding die is a method using typical ultra-fine processing technology such as precision machining technology by photolithography, which is a semiconductor production process, or a diamond tool. It becomes possible to manufacture the above-described optical members by molding plastics or glass using the molding die in which those processing technologies form the pattern.
However, in order to achieve the performance that the above-described optical element is required to have, very high precision is required in the size of a diffraction grating and a refraction-index value in a whole visible range of the material that constitutes the diffraction grating. On the other hand, when diffusion into general consumption products that are relatively cheap is preferred, it is desired that molding using plastic materials produce optical elements. However, when the plastic material is used, molding transcription and optical stability of the material itself become measuringly important. In particular, severe durability is required at the time of using them as lenses for cameras or the like, and hence it becomes indispensable to guarantee the optical property of material. However, as compared with glass, it is known that a refraction index of a plastic material changes significantly with a change in temperature, moisture content, etc. In particular, when diffraction optical elements are produced from plastic material, the plastic greatly affects their diffraction characteristics, and hence, there is a possibility of worsening the image quality output of an optical instrument. A proposed diffraction optical element using plastic materials has already been set forth by Japanese Patent Application Laid-Open No. 9-127322 but measures against the environment resistance such as the above-described temperature change, moisture content, etc., are not taken into consideration by this publication.
An object of the present invention is to provide a diffraction optical element that can solve the above-described problems of conventional technologies, can respond to changes in a surrounding environment, and whose perform hardly degrades even if it is used for a long time; and a production method thereof.
Another object of the present invention is to provide a diffraction optical element comprising: a first optical member having a first diffraction grating; a second optical member having a second diffraction grating, wherein the first and second optical members are stacked so that the first and second diffraction gratings faces to each other inside the stacked members and so that a space is formed between the diffraction gratings; and a sealing member for hermetically sealing the space between the diffraction gratings.
Still another object of the present invention is to provide a method of producing a diffraction optical element, comprising the steps of: staking a first optical member having a first diffraction grating and a second optical member having a second diffraction grating so that the first and second diffraction gratings faces to each other inside the stacked members and so that a gap is formed between the diffraction gratings; and hermetically sealing a space between the diffraction gratings with a sealing member.
A further object of the present invention is to provide an optical system comprising a plurality of optical elements including the diffraction optical element as described above.
A still further object of the present invention is to provide an optical apparatus forming an image by using the optical system as described above.